1. Field of the Invention
This invention relates to infrared dyes, to articles, in particular security documents, comprising such infrared dyes and to laser marking of articles containing these infrared dyes.
2. Description of the Related Art
Security cards are widely used for various applications such as identification purposes (ID cards) and financial transfers (credit cards). Such cards typically consist of a laminated structure consisting of various paper or plastic laminates and layers wherein some of them may carry alphanumeric data and a picture of the card holder. So called ‘smart cards’ can also store digital information by including an electronic chip in the card body. A principal objective of such security cards is that they cannot be easily modified or reproduced in such a way that the modification or reproduction is difficult to distinguish from the original.
Two techniques frequently used for preparing security documents are laser marking and laser engraving. In literature, laser engraving is often incorrectly used for laser marking. In laser marking, a colour change is observed by local heating of material, while in laser engraving material is removed by laser ablation.
Well known in the field of laser markable security documents is the use of laser markable polymeric supports. Laser marking produces a colour change from white to black in a laser markable support through carbonization of the polymer, usually polycarbonate as disclosed in e.g. EP-A 2181858 (AGFA GEVAERT).
During the past last years, there is an increased interest of using laser markable layers. The advantage of using a laser markable layer coated on a support instead of a laser markable support, is that a support can be used which has better physical properties than the laser markable supports, such as for example a higher flexibility than a polycarbonate support as disclosed in e.g. EP-A 2567825 (AGFA GEVAERT).
There is also an increased interest in using laser marking to produce coloured images in a security document. Therefore, laser markable layers are used which are composed of colour forming compounds (also called “leuco-dyes”) which can change from essentially colourless or pale-coloured to coloured when exposed to for example heat. Typically such a laser markable layer also contains an IR dye which absorbs the IR radiation and converts it into heat.
Multicolour images may be produced by using for example three different laser markable layers in one article, each laser markable layer comprising a leuco-dye generating a different colour and an IR dye having a different maximal IR radiation absorption so that each laser markable layer can be exposed separately. U.S. Pat. No. 4,720,449 (POLAROID) suggests infrared absorbers that absorb radiation at 760 nm, 820 nm and 1100 nm, and to use cyanine, merocyanine or thiopyrylium dyes that are substantially non-absorbing in the visible region of the electromagnetic spectrum so that it will not add any substantial amount of colour to Dmin areas, i.e. the highlight areas of an image. However, no examples of infrared dyes absorbing at 1100 nm are given.
In generating multicolour images with infrared lasers, so-called colour contamination or colour fogging may occur when the infrared absorption spectra overlap too much. Less overlap results in a larger achievable colour gamut. U.S. Pat. No. 4,720,449 (POLAROID) already disclosed that the infrared dyes should have absorption maxima that are at least about 60 nm apart. For a high colour gamut, it would be desirable to have larger gaps than 60 nm possible between the absorption maxima of the infrared dyes. However, there are two problems that limit the options for doing so.
A first problem is that dyes below about 830 nm tend to absorb also light in the visual spectrum from 400 to 700 nm, resulting e.g. in a white ID card have an annoying background coloration.
A second problem is that, although lasers above 1100 nm are available, infrared dyes with an absorption maximum above 1100 nm having minor or no absorption in the visual spectrum are not widely available. In the unpublished EP-A 1218814405 (filed on Nov. 10, 2012) infrared dyes are disclosed having an absorption maximum above 1100 nm. Some infrared dyes disclosed have an absorption maximum above 1150 nm.
There is still a need for infrared dyes having minor absorption in the visual spectrum and a maximum absorption above 1100 nm, so that the infrared absorption peaks of the different infrared sensitive recording layers can be spaced well apart in order to realize a multicolour laser markable article having a high colour gamut and minor background coloration.